The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 5,465,387; 5,899,991; 6,338,082; 6,393,468; 6,928,439; 7,031,984; 7,068,592; 7,403,925; 7,421,740; 7,555,482, 7,606,801 and 7,743,420; and
U.S. Published Patent Application Nos. 2003/0051026; 2004/0249847; 2005/0108206; 2005/0203881; 2005/0086529; 2006/0064313; 2006/0184530; 2006/0184459; 2007/0203872; 2007/0244899; 2008/0271157; 2009/0100058; 2009/0119298; 2009/0265780; 2011/0060916 and 2011/0061111.